This invention relates to an optical fiber test instrument with a mechanically positioned optical attenuator.
It is conventional to use an optical fiber to communicate an information signal from an optical driver at a proximal end of the fiber to an optical receiver at a distal end of the fiber. The optical driver includes a laser diode that generates pulses of optical power at a frequency that depends on the information signal and launches the pulses into the fiber at the proximal end thereof. Some of the optical energy that is launched into the fiber is returned to the proximal end of the fiber due to Rayleigh back-scattering. It is of interest to the manufacturer of an optical fiber to determine the variation with distance along the fiber of the intensity with which optical energy undergoes Rayleigh back-scattering. This may be achieved using an optical time domain reflectometer (OTDR).
The conventional OTDR shown in FIG. 1 comprises a laser diode 2 which emits light at, for example, 1310 nm and is optically coupled through a single mode fiber 4, a directional coupler 6, a second single mode fiber 8, and a front panel connector 10 to the proximal end of a fiber under test, or target fiber, 12, which also is a single mode fiber. Current pulses are applied to the laser diode by a pulse generator 14 through a laser driver amplifier 16 and cause the laser diode 2 to emit brief pulses of light that are launched into the target fiber 12.
Optical energy that is reflected and back-scattered within the fiber 12 is coupled through the front panel connector 10, the fiber 8, the directional coupler 6, and a multi-mode fiber 18 to a photodiode detector 20. The detector 20 generates a current signal depending on the power with which return optical energy is emitted from the fiber 18 at its proximal end. A first amplifier 24 converts the current signal provided by the detector 20 to a voltage signal, and a second amplifier 28 amplifies the voltage signal and applies it to an analog-to-digital (A/D) converter 32. The digital signal provided by the A/D converter is processed by a processor 36, which is used to provide a display on a display device 40, e.g. a cathode ray tube, of the level of return power as a function of distance. Operation of the reflectometer is controlled by a controller 44.
The dynamic range of the optical signal received at the detector of an OTDR is enormous. In general, the intensity of the optical signal depends upon the distance along the fiber at which the back-scattering took place: the intensity of the back-scattered energy decreases with distance from the proximal end of the fiber under test. Therefore, when using the OTDR to examine a range that is close to the proximal end, the intensity of the back-scattered energy is higher than when examining a range that is far from the proximal end.
In order for the digital values provided by the A/D converter 32 to represent accurately the level of return power received at the detector 20, it is necessary that the detector not be overloaded by the return power, that amplifiers 24 and 28 not be saturated, and that the voltage signal applied to the A/D converter be within the converter's range.
U.S. Pat. No. 4,960,989 (Liebenrood et al) discloses an OTDR in which the gain of the detector is adjusted as a function of time relative to pulsing of the laser diode in order to prevent overdriving of the first amplifier. U.S. Pat. No. 4,960,989 is not concerned with the variation in power level due to change in intensity of Rayleigh back-scattering but with masking the effect of reflections, for example at connections between lengths of fiber and at breaks in the fiber.
It has also been proposed that the intensity of back-scattered light received at the detector should be controlled by controlling the current that is used to drive the laser diode. However, as the current is varied, the spectral characteristics of the light emitted by the laser diode also vary, and therefore the measurement results are not accurate.